Radioactive metal products and methods of making same



Patented Feb. 22, 1949 RADIOACTIVE METAL PRODUCTS AND METHODS OF MAKING SAME poration of Delaware No Drawing. Application November 26, 1946, Serial No. 712,250

14 Claims.

This invention relates to the manufacture oi radioactive metal products, and provides an improved mixture of finely divided gold and a finely divided radioactive substance, and an improved method for making it. The method of the invention involves preparing the mixture by precipitation (advantageously as a co-precipitate) from a solution containing both gold and a radioactive substance in solution. The finely divided mixture, when prepared as a co-precipitate in accordan; with the invention, is a new product in which the radioactive material is for the most part securely bound to the gold particles and cannot be separated therefrom to any very 1 great degree by repeated decantation washing with water.

Radioactive metal products comprising a finely divided radioactive substance intimately dispersed throughout a mass of metal (as described, for example in United States Patent No. 2,326,631, to Alois Fischer), have proved to be extremely convenient for many uses as sources of radioactive energy. Wires, sheets, foils or specially formed articles may readily be made from such radioactive metal compositions for use medicinally, to illuminate phosphors, in static eliminating devices, and for many other purposes where radioactive energy is of value.

These radioactive metal products are most advantageously prepared by compacting and sintering a mixture of a metal powder and a finely die vided radioactive substance. The sintered compact may then be rolled, drawn otherwise fabricated. Numerous metal powders may be used in making the product, but gold powder is generally the most satisfactory and is most com-. monly used. While gold is generally considered an expensive metal, its cost is low in relation to, the cost of the commonly used radioactive substances (e. g., radium salts), and in relation to the amount of work usually involved in making and fabricating the radioactive metal product. Moreover, its physical properties make it a particularly desirable metal for use in these products. It will not oxidize or tarnish even in corrosive atmospheres; it is the most ductile and malleable of all metals, so that it is easily worked into almost any desired shape; and it is veryeasily amenable to fabrication by the techniques of powder metallurgy.

Heretofore radioactive gold products have commonly been produced by mechanically mixing commercial gold powder with a radioactive substance, such as radium sulphate in the desired pr po t on he. m x powders then are,

pressed, sintered and worked to final form. Preparing the mixture of gold powder and radioactive substance by mechanical mixing is a difficult and hazardous operation. In the first place, it generally involves precipitating and filtering radium sulphate or other radioactive substance in substantially pure form, and ashing the filter paper. The latterstep particularly requires careful control to avoid reducing any of the radium sulphate to radium sulphide, which is soluble and may be leached out of a metallic article in which it is incorporated. In the second place, there is always the danger that some of the costly radium sulphate or other radioactive material will be lost in the course of transferring it to the mixing vessel wherein it is admixed with the gold powder. In the third place, the operation must be thorough and carefully controlled so that the radioactive substance be comes uniformly distributed throughout the mass of gold powder. In the fourth place, carrying out the foregoing steps involves considerable health hazard because. of the poisonous nature of radioactive materials, and the possibility of exposure to radioactive dust.

The present invention provides a rapid and convenient method for preparing the improved mixture of gold powder and radioactive material in accurately controllable amounts and proportions, without the difficulties and hazards involved in heretofore known procedures for preparing mixtures of these components. The new method involves first preparing a solution containing both a reducible gold salt and a soluble salt of a radioactive element, then adding to the solution a substance capable of reducing the gold salt in solution to metallic gold and a substance capable of forming an in soluble compound of the radioactive element, thereby forming a mixed precipitate of metallic gold and radioactive compound, and subsequently separating the mixed precipitate from the resid-. ual solution. Advantageously the substance capable of reducing the gold compound and the substance which forms the insoluble radioactive compound are added together in a single reagent, thus causing co-precipitation of the gold and radioactive substance. Such co-precipitation results in a particularly advantageous product wherein the radioactive material is securely bound to the gold particles and cannot be washed therefrom with water. In some cases, however, it may be advisable to add these substances separately to the solution, and the invention contemplates. their addition in this manner.

finally dissolving the resulting gold chloride in water. 7

Auric chloride is the preferred gold chloride to employ, and in fact there is some doubt as to Whether other gold chlorides are stable in water solution. However, it is not essential that gold chloride be employed as the reducible gold salt. Gold salts generally are characterized by being easily reduced in solution and any gold salt that is thus reducible and is reasonably stable in water solution may be employed. For example, gold (preferably auric) bromide may be used, as may auric sulphate or aurous hydroxide. The technique of preparing and handling these other soluble gold salts is rather difficult, however, and for this reason they are not preferred, although they are included within the scope of the invention.

A soluble salt of a radioactive material is incorporated in the gold salt solution. In general the most satisfactory radioactive material to employ is radium in the form of the chloride or bromide, both of which are water soluble. Advantageously a solution containing the desired amount of radium chloride or radium bromide is added to the gold salt solution, but it is quite feasible to dissolve the crystalline radium salt directly in the gold solution.

Radioactive materials other than radium may, of course, be employed if it is desired to make up a product containing some other radioactive material. For example, the naturally occurring radioactive materials, such as polonium, actinium, thorium or uranium, may be employed, in which case a soluble salt of the element selected is dissolved in the gold solution. It is also possible to employ radioactive materials higher in the periodic system than uranium, such, for example,

as neptunium, plutonium, americium, and curium, soluble salts of which are known and can be incorporated in the solution of gold chloride or other gold salt. Similarly, artificially radioactive substances, such as radioactive isotopes of copper, nickel, iron, sodium, phosphorus, chlorine or iodine, may be employed. When such artificially radioactive material is used, a soluble salt of the radioactive element, such as the chloride of radioactive copper or sodium iodide containing radioactive iodine, is dissolved in the gold solution.

After preparing the solution containing both the reducible gold salt and a salt of the radioactive element in the desired quantities and relative proportions, the solution is treated with one or more agents capable of precipitating metallic gold from the solution and of forming an insoluble compound containing the radioactive element. In the preferred case employing a solution containing gold chloride and a soluble radium salt, it is preferred to co-precipitate the gold and radium by means of ferrous sulphate. Ferrous ions in solution are capable of reducing gold from the chloride in accordance with the following simple oxidation-reduction reaction:

At the same time, the sulphate ions introduced into solution with the ferrous sulphate precipitate radium from solution in the form of insoluble radium sulphate, as follows:

The co-precipitate prepared in this manner is an extremely intimate admixture of metallic gold and radium sulphate. Provided only that sumcient ferrous sulphate is added, the precipitation of both the gold and radium is virtually complete, so that the co-precipitate contains essentially all of the gold and radium .originally present in the solution, and in the proportions in which they were added to the solution. Experiments have shown that when the co-precipitation is properly carried out, recovery of both gold and radium in the co-precipitate is complete to the extent that can be determined by ordinary analytical procedures. Mesothorium I (atomic weight 228, atomic number 88) co-precipitates with gold in the same fashion as radium.

The product obtained by co-precipitation of the gold and insoluble radium compound with ferrous sulphate or other co-precipitating reagent is unique in that the radioactive substance is securely bound to the gold particles. While it has not been possible to demonstrate the nature of this bond, we believe that a large proportion of the precipitated particles of the radioactive substance become mechanically interlocked with and embedded in the finely divided co-precipi tated gold particles. Whatever the nature of the bond, however, the co-precipitated product is one from which the radioactive substance cannot be separated to a very substantial degree by repeated decantation washing with water; The new product therefore is one from which there is no likelihood that the radioactive substance will be washed out if the product made from the coprecipitated materials becomes wetted or is washed. In this respect the new co-precipitated product difiers from heretofore known mixed gold and radioactive powders such, for example, as described in Fischer Patent No. 2,326,631 above mentioned. For example, a very large proportion (half or more) of a radium compound may be separated by repeated decantation washing from a mixture of such compound with gold powder prepared by mixing the two finely divided materials in the dry state. Even when the mixture has been prepared by successively precipitating the gold and radium compound from a single solution, a large part of the precipitated radium compound may subsequently be separated from the gold powder by repeated decantation washing. When the product has been prepared by co-precipitation of the gold and radium sulphate or other radioactive material, however, a total of less than about 20% of the radioactive material will be separated by repeated decantation washings continued to the point where the final washings remove only barely measurable amounts of the radioactive substance.

While ferrous sulphate constitutes the preferred precipitating reagent for use when gold chloride and a soluble radium salt are dissolved in the initial solution, it is not essential that this particular precipitant be employed. Gold chloing agents such as oxalic acid, sodium oxalate,

or materials in the class of photographic developers, for example, pyrogallol, hydroquinone, or glycine. acid also have been used successfully to reduce and precipitate the gold.

Likewise, some other precipitant than sulphate ions may be used to precipitate the radioactive substance. Radium carbonate, for example, is quite insoluble, so that a reagent containing carbonate ions in place of sulphate ions may be used to precipitate radium with gold. If some radioactive material other than radium is employed in the solution, any appropriate precipitant for that material may be used. Polonium sulphide, for example, is insoluble, and this radioactive material may be precipitated by a reagent including sulphide ions. Similarly, artificially radioactive copper or iron may be precipitated as sulphide, carbonate, hydroxide, or other insoluble compound by the use of a reagent including sulphide ions, carbonate ions, hydroxyl ions, or the like. If the radioactive material is present in solution as an anion, as, for example, if radioactive iodine in the form of sodium iodide isemployed, it may be precipitated as insoluble mercuric iodide 01' insoluble silver iodide by a reagent yielding mercuric or silver ions.

It is usually preferred, if practicable, to employ a single reagent including the precipitants for both the gold and the radioactive compound (such as a solution of a salt-e. g., ferrous sulphate-of which the cation is capable of reducing the gold in solution and of which the anion is capable of forming an insoluble compound with the radioactive element but not with the gold), as then the gold and radioactive substance will be co-precipitated, with the advantages described above. The precipitating agents may, however, comprise two separate materials, one of which serves primarily to reduce the gold, and the other of which serves to precipitate the radioactive substance, and they may be added separately one after the other, or concurrently, as is best suited to the requirements of the particular case. For example, a gold reducing agent such as stannous ions, mercurous ions, or oxalic acid, and a substance such as sodium sulphate or sodium carbonate capable of forming an insoluble salt with a radioactive material such as radium, may be used to effect precipitation. If such separate reagents are used concurrently, the benefits oi co-precipitation will be secured.

In some cases it may be advisable to add separate precipitants separately to the solution. For example, if the radioactive material (polonium, for example) is to be precipitated as the sulphide, it is best to precipitate the gold first (say by means of ferrous sulphate) and then to incorporate sulphide ions in the solution to precipitate the polonium, which thereupon becomes mixed with the previously precipitated gold. In this way there is no danger that any undesirable gold sulphide will be precipitated. Generalizing on this example, it is ordinarily best to add the two precipitants separately if either one is likely to impair the quality of the precipitate which the other is intended to produce, and to add first, in such case, the precipitant that will not inter- Phosphorus acid and hypophosphorus 6 fere with. later proper; action. of the; other precipitant.

In some cases. the reagent may comprise a single. substance which servesboth to reduce the gold; and precipitate. the radioactive substance for example, metallic gold and radioactive iodine in. the. form of a soluble iodide may both be precipitated from solution. by a reagent contain ing mercurous ions, which. serve both to reduce the gold and: also to form insoluble mercury iodide.

The preparation of a mixed precipitate of gold and polonium sulphide provides. an example 11- lust-rating the modifications outlined above. To prepare. such a mixed precipitate in accordance with. the invention, a solution containing a soluble reducible gold compound (say auric chloride) and a soluble compound of polonium (say polonium. chloride) is made up to contain the gold and polonium in the same relative proportions that are desired in the final mixed precipitate. In addition, a small quantity of a soluble bismuth or lead salt is dissolved in the solution to provide a carrier for the polonium when the latter is precipitated. In place of bismuth or lead, some other substance that will form an effective carrier precipitate for the polonium precipitate may be used instead. Ferrous ions then are incorporated in the solution (as by adding to it a solution of ferrous sulphate), slightly in excess of the amount required to precipitate the gold. After the gold has been precipitated, but before separating it from the solution, the polonium is precipitated as the sulphide by treatment with hydro-gen sulphide. For purposes of this treatment, the acidity of the solution is first adiusted to a value (ordinarily a pH value of about 0.5) at which-- ferrous ions do not precipitate with hydrogen sulphide, and hydrogen sulphide is then passed into the solution. The small quantity of lead or bismuth in solution precipitates as the sulphide under these conditions, and this precipitate serves to collect and carry with it con-- currently formed insoluble polonium sulphide, which is usually very small in amount. The solution may be stirred during treatment with the hydrogen sulphide, so as to keep the precipitated gold in. suspension and insure effective mixing with the sulphide precipitate. After completion of the sulphide precipitation, the mixed precipitate of metallic gold and polonium sulphide (with such other carrier sulphide as may be present) is allowed to settle, and may then be separated by decantation or filtration from the residual solution. The separated precipitate may be washed, after which it is further treated in essentially the same manner as the precipitate of gold and radium sulphate.

It is evident from the foregoing that there is wide latitude in the choice of suitable precipitating agents, and that the choice ordinarily will depend' upon the particular radioactive material dissolved in the initial solution. In general, it is necessary only that the precipitating agent or agents comprise, or produce in the solution, a reducing agent for the gold salt, and a substance capable of forming an insoluble compound with the radioactive material present.

Aft-er precipitating the gold and radi r-active substance (radium sulphate, for exampie), the mixed precipitate or co-precipitate is separated from the residual solution by decantation, filtration or other suitable means, and is dried either at atmospheric temperature or at some suitably elevated drying temperature.

An advantage of the new method is that the precipitate may be filtered and may be separated from the filter paper substantially completely while still wet. Little or none of either the gold or radium sulphate (or other radioactive cornpound) settles into the pores of the filter paper. In consequence the filter paper need not be ashed, and the attending danger of reducing any radium sulphate to the sulphide is avoided. The mixed precipitate dries easily and quickly at room temperature after being separated from the filter paper.

In the case of gold and radium sulphate coprecipitated by means of ferrous sulphate, the dried coprecipitate contains the radium sulphate or other radioactive material in sufiiciently intimate admixture with the gold so that no further mixing ordinarily is required. It may, however, be ground to break up clumps and to improve the uniformity of the mixture. If gold and radioactive substance have been precipitated by separately added precipitating agents, it generally is advisable to grind the mixed precipitate (before or after drying) to insure formation of an intimate admixture of the two components.

The dried mixed precipitate may be introduced into a compacting die for compacting into the form of a briquette by the application of mechanical pressure, and the resulting compact then may be sintered by heating to a temperature approximately 950 C., as described in the abovementioned Fischer Patent No. 2,326,631, or as described in the copending application of Willhausen, Dooley and Carroll, Serial No, 654,460, filed March 14, 1946. After the compact has been sintered, it may be mechanically worked by rolling, drawing, or otherwise to form from it a metallic article of any desired form or shape suitable for the use to which it is to be put.

A very real advantage resulting from preparing the finely divided gold by precipitation from solution with ferrous sulphate is that the particle size and form of the precipitated gold is excellent for subsequent compressing and sintering of the mixture into a metallic article by the methods of powder metallurgy.

Another advantage of preparing the compact from gold powder that has been precipitated from a chloride solution by ferrous sulphate is that the gold powder so prepared is of high purity and may be introduced directly into a sintering furnace already heated to the sintering temperature of about 950 C. Compacts prepared from ordinary commercial gold powders cannot be directly introduced into a furnace heated to such a high temperature, because the large volume of gas evolved in a short period of time when such powders are heated rapidly causes blisters to be raised on the surface of the sintered compact, and further working of the compact is made very difiicult.

Still another advantage of the precipitated gold powder prepared by precipitation with ferrous sulphate is that when dried it does not tend to stick to the compacting die. The compact is readily removed from the die with virtually no loss due to small pieces of the compacted powder adhering to the die. Similarly there is no tendency for the gold powder so prepared to stick to the Walls of the vessels in which it is handled prior to introducing it into the compacting die. This characteristic is of special advantage if it is desired to grind the mixed precipitate before sintering.

The preparation of mixtures of gold powder and radium sulphate or other radioactive material may be eflected very rapidly by the new method, and the mixture may be converted to the form of a sintered compact in a minimum length of time. Consequently, the hazards of over-exposure to radioactive radiations, always present in handling concentrated radioactive materials, is reduced to a minimum. Moreover, since the radium sulphate or other radioactive material admixed with the gold is not first isolated in the pure form, the danger of losing any of this costly material by having small particles blown away by air currents, or by other causes of losses of small particles. is greatly reduced.

We claim:

1. The method of preparing a mixture of metallic gold and a radioactive compound which comprises preparing an aqueous solution of a reducible gold salt and a water-soluble salt of a radioactive element, adding to the solution a substance capable of reducing the gold salt in solution to metallic gold and a substance capable of forming an insoluble compound of the radioactive element thereby forming a mixed precipitate of metallic gold and insoluble radioactive compound, and separating said mixed precipitate from the residual solution.

2. The method of preparing a mixture of metallic gold and a radioactive compound which comprises preparing an aqueous solution of gold chloride and a water-soluble salt of a radioactive element, adding to the solution a substance capable of reducing the gold chloride in solution to metallic gold and a substance capable of forming an insoluble compound of the radioactive ele-" ment, thereby forming a mixed precipitate of metallic gold and insoluble radioactive compound, and separating said mixed precipitate from the residual solution.

3. The method of preparing a mixture of metallic gold and a radium compound which comprises preparing an aqueous solution of a reducible gold salt and a water-soluble radium compound, adding to the solution a substance capable of reducing the gold salt in solution to metallic gold and a substance capable of precipitating an insoluble radium compound, thereby forming a mixed precipitate of metallic gold and insoluble radium compound, and separating said mixed precipitate from the residual solution.

4. The method of preparing a mixture of metallic gold and a radium compound which comprises preparing an aqueous solution of gold chloride and a water-soluble radium compound, adding to the solution a reagent capable of reducing the gold chloride in solution to metallic gold and of precipitating an insoluble radium compound, thereby forming a co-precipitate of metallic gold and radium compound, and sepa-- rating said co-precipitate from the residual solution.

5. The method of preparing a mixture of metallic gold and a radium compound which comprises preparing an aqueous solution of gold chloride and a water-soluble radium salt, concurrently incorporating in the solution a substance yielding ferrous ions and ions capable of forming an insoluble compound with radium, thereby forming a co-precipitate of metallic gold and radium compound, and separating said 00- precipitate from the residual solution.

6. The method of preparing a mixture of metallic gold and radium sulphate which comprises adding ferrous sulphate to an aqueous solution containing both gold chloride and aqueous water-soluble radium salt, thereby co-precipitating metallic gold and radium sulphate, and sepang the resulting co-precipitate from the nz-sidual solution.

"I. lhe method of preparing a radioactive metal product comprising gold and a radioactive comnd intimately dispersed therein which comrlses preparing an aqueous solution of a reib-le gold 'salt and a water-soluble salt of a ioactive element, adding to the solution a stance capable of reducing the gold salt in lticn to metallic gold and a substance capable forming an insoluble compound of the radioive element, thereby forming a mixed precipie of metallic gold and insoluble radioactive 1130111151, separating said mixed precipitate a the residual solution, drying the mixed pre pitate, subsequently compressing and sinterthe mixed precipitate into a compact mass, and working the resulting compact into a product of desired form.

8. The method of preparing a radioactive metal product comprising gold and a radium compound in intimate admixture which comprises preparing an aqueous solution 01" gold chloride and a water-soluble radium salt, concurrently adding to the solution a substance yielding ferrous ions and ions capable of forming an insoluble compound with radium, thereby forming a co-precipitate of metallic gold and radium compound, separating said co-precipitate from the residual solution, drying the co-precipitate, subsequently compressing and sintering the co-precipitate into a compact mass, and working the resulting compact into a product of desired form.

9. The method of preparing a radioactive metal product comprising gold and a radioactive compound which comprises precipitating metallic gold and a radioactive compound mixed together from an aqueous solution having both a gold compound and a radioactive substance dissolved therein, by adding to the solution at least one reagent which reduces the gold compound to metallic gold and which forms a water-insoluble compound with the radioactive metal.

10. The method of preparing a mixture of finely divided gold and radioactive compound in predetermined amount and proportions which comprises preparing an aqueous solution having dissolved therein a gold compound an an amount corresponding to the amount of gold desired in said mixture and a radioactive substance in an amount corresponding to the amount of radioactive compound desired in the mixture, adding to the solution a reagent capable of precipitating therefrom a mixed precipitate containing substantially all of the gold in metallic form and substantially all of the radioactive substance in the form of a water-insoluble radioactive compound, and separating the mixed precipitate from the resulting solution.

11. The method of producing a mixture of finely divided metallic gold and a finely divided water-insoluble radioactive compound in which the radioactive material is for the most part securely bound to the gold and is not separated therefrom to any substantial degree by repeated deoantation washing with water, which comprises co-precipitating the gold and radioactive compound from an aqueous solution of a reducible gold salt and a water-soluble salt of a radioactive element by adding to such solution a reagent capable of simultaneously reducing the gold salt in solution to metallic gold and precipitating the insoluble radioactive compound, and separating the resulting co-precipitate from the residual solution.

12. The method of producing a mixture of metallic gold and polonium sulphide which comprises preparing an aqueous solution of a reducible gold salt and a water-soluble polonium compound, adding to the solution a substance capable of reducing the gold salt in solution to metallic gold, thereafter adding to the solution an aqueous solution yielding sulphide ions under conditions such as to cause precipitation of polonium sulphide, thus forming a mixed precipitate of gold and polonium sulphide, and subsequently separating the mixed precipitate from the residual solution.

13. The method of producing a mixture of metallic gold and polonium sulphide which comprises preparing an aqueous water-solution containing a reducible gold salt, a water-soluble polonium compound, and a soluble salt of a metal capable of forming an insoluble sulphide carrier for the polonium, adding to the solution an aqueous solution yielding ferrous ions to reduce the gold salt in solution to metallic gold, adjusting the acidity of the solution to a value such that ferrous sulphide will not precipitate upon treatment with hydrogen sulphide, thereafter treating the solution with hydrogen sulphide so as to cause precipitation of the sulphide carrier and of the polonium, thus forming a mixed precipitate of gold, sulphide carrier, and insoluble polonium compound, and subsequently separating the mixed precipitate from the residual solution.

14. A finely divided radioactive metallic gold preparation produced in accordance with the method set forth in claim 1, characterized in that the radioactive compound is for the most part securely bound to the gold particles and is not separated therefrom to any substantial degree by repeated decantation washing with water.

CLARENCE W. WALLHAUSEN. HARRY H. DOOLEY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,826,631 Fischer Aug. 10, 1943 OTHER REFERENCES Mellor: Inorganic and Theoretical Chemistry,

vol. 3, page 602, and vol. 4, pages 93, 114 and 115.

Certificate of Correction Patent No. 2,462,241. February 22, 1949. CLARENCE W. WALLHAUSEN ET AL.

It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 4', line 3, extreme right portion of the first equation, for Au read Aul in the third equation, same column, for that portion thereof reading 3RaSO read SRaSOA column 7, lines 33 and 34, for the name Willhausen read Wallha'usen; column 8, line 75, claim 6, for the word aqueous read a; column 9, line 51, claim 10, for an, first occurrence, read in; column 10, line 30, claim 13, before soluble insert the word and hyphen water-;

and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Ofiice.

Signed and sealed this 21st day of June, A. D. 1949.

THOMAS F. MURPHY,

Assistant Oommissz'oner of Patents.

Certificate of Correction Patent No. 2,462,241. February 22, 1949. CLARENCE W. WALLHAUSEN ET AL.

It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 4", line 3, extreme right portion of the first equation, for Au read Aul in the third equation, same column, for that portion thereof reading 3RaSO read 8RaS0 l column 7, lines 33 and 34, for the name Willhausen read Wallha'use'n; column 8, line 75, claim 6, for the word aqueous read a; column 9, line 51, claim 10, for an, first occurrence, read in; column 10, line 30, claim 13, before soluble insert the word and hyphen water-; and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 21st day of June, A. D. 1949.

THOMAS F. MURPHY,

Assistant Commissioner of Patents. 

